1. Field of the Invention
The present invention relates to an ultraviolet detector which detects ultraviolet radiation incident thereon by converting them into an electric signal.
2. Related Background Art
An example of conventional ultraviolet detectors is disclosed in Japanese Utility model Publication No. 49-17184. This publication discloses an ultraviolet detector in which an anode and a cathode are disposed within a sealed vessel constituted by a glass envelope and a glass bottom plate welded to the bottom portion of the glass envelope.
Though the conventional ultraviolet detector mentioned above is an excellent detector which has a long life and can stably detect ultraviolet radiation, its characteristics may not be sufficient. Specifically, when used for a long period of time, its characteristics may deteriorate over time, thus lacking in stability.
In order to overcome such shortcomings, various studies have conventionally been made. The inventors have elucidated that these shortcomings result from the glass material used as a window material for the ultraviolet detector. Typical glass materials which are transparent to ultraviolet radiation contain fluorine. Upon welding of the envelope and bottom plate of the ultraviolet detector, fluorine contained in the glass material evaporated from the glass material and adsorbed onto the surfaces of the anode and cathode, the inner surface of the sealed vessel, and the like. Normal operation of the detector and the aging process in fabrication both include the gas discharge between the electrodes. Electrons and ions generated by the gas discharge impinge onto the surfaces of the anode and cathode respectively. It causes the desorption of fluorine adsorbed on the surface of these electrodes. The fluorine containments on the other sites in the vessel can also be desorbed by means of the heat which arises in the aging processes of the detector fabrication and even in the normal operation condition of the detector. The desorbed fluorine alters the ionization property of the discharged gas filled in the vessel. This alternation commonly results in the lowering of the breakdown voltage and that leads to occasional and continuous false discharges and unwanted increase of the sensitivity. These effects considerably degrade the stability and the reliability of the detector.
In order to overcome the foregoing shortcomings resulting from the use of such a glass material, it is an object of the present invention to provide an ultraviolet detector having characteristics which are better than those conventionally attained.
The ultraviolet detector in accordance with the present invention comprises a sealed vessel, an anode, a cathode, a lead pin and a gas enclosed in the sealed vessel. The sealed vessel includes a tubular member having an opening and being made of a metal material blocking ultraviolet radiation, a window member being made of a glass material transparent to ultraviolet radiation and closing aforementioned opening and a stem having a metal portion contacting to the tubular member and a glass portion not contacting the tubular member. The anode is disposed within the sealed vessel at positions opposing said window member by the lead pin which penetrates the glass portion of the stem for supplying voltage. The cathode is disposed within the sealed vessel between the window member and the anode and secured to the tubular member or the metal portion of the stem.
In such a configuration, since the tubular member is made of a metal material blocking ultraviolet radiation, incident ultraviolet radiation are introduced through the window member made of an ultraviolet-transparent material toward the anode and cathode of the detector, whereby the detector exhibits a high directivity. Further, since the tubular member is made of a metal material, even when this tubular member is connected to the metal portion of the stem by pressure or welding, impurities such as fluorine do not attach to the sealed vessel, anode, and cathode. Accordingly, the ultraviolet detector in accordance with the present invention is prevented from being affected by fluorine or the like, whereby the break down voltage of the detector can be held stably.
And more, the cathode of the present invention is secured to the tubular member or the metal portion of the stem without a stem pin. So it is easy to manufacture the ultraviolet detector having discharging gap with a high precision.
According to the present invention, the cathode may be integrated with the tubular member or the metal portion of the stem may be a ring shaped rim of the stem.
Such configuration aids in facilitating manufacture of high accurate ultraviolet detector.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.